1. Technical Field of the Invention
The present invention relates generally to an improvement on an oxygen sensing element which may be used in an oxygen sensor installed in an exhaust system of an internal combustion engine for automotive vehicles to measure an oxygen content in exhaust gases.
2. Background of Related Art
For burning control of fuel, an air-fuel ratio sensor is installed in an exhaust system of an engine for automotive vehicles to measure an air-fuel ratio. Such an air-fuel ratio sensor is usually implemented with an oxygen sensor having disposed therein an oxygen sensing element made of a solid electrolyte having an oxygen ion conductivity.
Such an oxygen sensor device is known in the art consists of a cup-shaped solid electrolyte body having an atmospheric chamber defined therein, an outer electrode disposed on an outer surface thereof, and an inner electrode disposed on an inner surface thereof exposed to the atmospheric chamber. The structure also has a ceramic heater installed in the atmospheric chamber to heat the oxygen sensing element up to an activating temperature quickly.
The outer electrode is formed by applying a conductive paste made of noble metallic powder dispersed in an organic binder to the outer surface of the solid electrolyte body and baking the conductive paste.
The outer electrode may alternatively be formed using another technique such as chemical plating, chemical vapor deposition, or physical vapor deposition.
In recent years, the oxygen sensing element having the electrodes formed in the manner, as described above, has been highlighted for high output and found to be suitable for use in oxygen sensors which are required to provide high outputs, especially within a low temperature range below 400xc2x0 C.
Usually, a conductive paste is baked at 1400 to 1500xc2x0 C., while a chemically plated layer is baked at 800 to 1000xc2x0 C. The chemical plating, thus, realizes a high-output sensor with improved catalytic activity. The reason why the baking temperature of the conductive paste is higher than that of the chemically plated layer is that it is necessary to bake platinum powder having a grain size of the order of 0.5 to 1.0 xcexcm.
The outer electrode with the improved catalytic activity formed using the chemical plating has, however, the following drawbacks.
FIG. 4 illustrates a relation between outer surface temperature measured at a portion of a solid electrolyte body of an oxygen sensing element and distance to the portion of the solid electrolyte body from the head thereof. The relation shows that the outer surface temperature of the solid electrolyte is substantially constant around the head thereof, while it is decreased suddenly over a given boundary to a base of the solid electrolyte body. This is because a portion of the oxygen sensor element facing a ceramic heater, as shown in FIG. 1, is heated uniformly, while the other portion apart from the ceramic heater is lower in temperature. Thus, the output from the base of the outer electrode formed in the chemical plating is, as can be seen from the drawing, higher than that from an electrode made of a conductive paste in both cases where the thickness of the outer electrode is 0.5 xcexcm and 1.5 xcexcm. The total output from the outer electrode is actually the sum of outputs from the head and base thereof, but the output from the base of the outer electrode will be lower than that from the head when the base is smaller in thickness than the head, so that the output from the oxygen sensing element may be insufficient for functioning properly.
In recent years, the temperature at which the oxygen sensing element is used is more increased, resulting in exacerbation in the above problem. Specifically, an increase in ambient temperature will cause a difference in temperature between the head and base of the oxygen sensing element to be increased, resulting in a great difference in outputs therebetween. The total output from the oxygen sensing element, thus, depends greatly upon the output from the base thereof.
The use of a ceramic heater producing a large quantity of heat enough to raise the temperature the whole of the outer electrode uniformly may be proposed, but it may cause the overall temperature of the oxygen sensing element to be increased more than the temperature a filter, leads, and heater solder can withstand.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an improvement on an oxygen sensing element capable of measuring an oxygen content in gasses with high accuracy.
According to one aspect of the present invention, there is provided an oxygen sensing element for measuring an oxygen content in gases which comprises: (a) a cup-shaped solid electrolyte body having a portion exposed to the gases which has a given length; (b) an inner electrode formed on an inner wall of the solid electrolyte body to provide an output signal; and (c) an outer electrode formed on an outer wall of the solid electrolyte body to provide an output signal for determining the oxygen content in the gases based on the output signals from the inner and outer electrodes. The outer electrode occupies an area on the outer wall of the solid electrolyte body within a range of 80% of the given length of the gas-exposed portion of the solid electrolyte body and has a thickness ranging from 1.2 to 3.0 xcexcm.
In the preferred mode of the invention, a second outer electrode is further provided which is formed on a portion of the outer wall of the solid electrolyte body adjacent to the outer electrode and which has a thickness smaller than that of the outer electrode.
The thickness of the second outer electrode is within a range from 0.3 to 2.4 xcexcm.
The ratio of the thickness of the second outer electrode to the thickness of the outer electrode is within a range of 0.25 to 0.8.